1. Field of the Invention
This invention relates to a semiconductor device in which a semiconductor substrate is interfaced with a dissimilar material which is generally incompatible therewith.
2. Discussion of Background
Semiconductor technology is in large measure based on material structures made up of dissimilar materials interfaced to form composite structures. Each material contributes to the overall performance of the structure in ways that depend on the basic properties of the particular material and the way in which the structure has been designed to use those properties. In addition, these structures very often have performance characteristics that depend not only on the basic properties of the individual materials, but also on interactions between the individual materials at their interfaces. These interfacial (interactive) effects can dominate the performance of a given materials combination. In those circumstances, even though the individual material properties suggest that the materials can be combined to perform a useful function, nevertheless interfacial effects may cause the combination to perform in an unpredicted and very often non-useful manner.
If other materials can be chosen to perform the same function, and, if these combinations do not exhibit undesirable interfacial (interactive) effects, then the problem is solved. There are, however, important cases where no alternative materials combinations have been demonstrated. In these cases individual materials with the correct properties are known, but they have yet to be combined without substantial compromise of the expected performance due to undesirable interfacial (interactive) effects in the composite structure.
One example of the above problem is found in an important class of electronic devices which depend on modulation of the surface potential of a semiconductor to control the operation of an electronic device. This group of devices includes, but it is not limited to, Field Effect Transistors (FETs) (of many types) Metal Insulator Semiconductor (MIS) capacitors, and Charge Coupled Devices (CCDs). In these applications control of the surface potential is an active component of the device operation.
Another class of electronic devices requires control of the surface properties of a semiconductor in a different sense. In these devices control of the surface potential does not play an active role in the device operation, but it does play an important role in the stability and optimization of the device performance characteristics. This class of devices requires passivation of one or more interfaces of the active device. This category includes but is not limited to bipolar transistors (of many types), where uncontrolled changes in the properties of the surface lead to undesirable changes in device performance. These applications depend on stabilization of the surface properties of the semiconductor over the entire range of fabrication and operating conditions. This is termed surface passivation.
Individual devices may require both active and passive control of the surface potential. This is true in a Metal Oxide Semiconductor Field Effect Transistor (MOSFET).
Another class of applications occurs when multiple devices are integrated in the same block of semiconductor material. Areas of the surface of that semiconductor not occupied by active elements need to be similarly passivated to ensure operational stability of the adjacent active devices. This is characteristic of the so called "field" areas of an integrated circuit.
For each of the applications outlined above, silicon can be directly oxidized to form silicon dioxide. Silicon dioxide forms an excellent interface with silicon. The properties of the silicon/silicon dioxide interface are suitable for both active applications, such as gate insulator structures for FETs, as well as passive applications, such as discrete device or circuit passivation. This fact has been critically important in the development of the silicon-based electronics industry.
For some time substantial interest and activity has centered around other semiconductor materials, such as Ge, GaAs and its alloys, and other III-V compounds such as InSb and InP. These materials offer performance advantages with respect to silicon or can serve in applications where silicon can not. Unfortunately these materials, of which GaAs is illustrative, have not been passivated by oxidation or other direct chemical reactions of the host semiconductor surface. For these materials oxidation or similar processes that react the semiconductor constituent(s) lead to unstable interfacial properties.
Several factors can contribute to this instability. For example in GaAs the oxides of Ga and As are chemically unstable under normal conditions experienced during fabrication or operation of the device. In addition to these chemical instabilities these oxides do not have electrical properties comparable to those of silicon dioxide. These properties include resistivity, bulk trap density, interface state density with the parent material, breakdown field, radiation induced trapping, wearout and other hot carrier effects. Thus these oxides can not be used as gate insulators for higher performance FET structures. These problems are characteristic of the oxides themselves, independent of the technology used to form the oxides. Along with the poor performance of these oxides for electronic applications, the oxidation process degrades the surface of the GaAs. In particular, arsenic is preferentially removed from the GaAs surface during oxidation. This typically results in a thin layer of elemental arsenic at the interface between the oxides and the semiconducting GaAs. The arsenic deficient GaAs surface has electronic properties that will not allow many types of devices to work, particularly MISFETs (metal Insulator Semiconductor Field Effect Transistors).
In spite of substantial research efforts over the years, up to and including the present time, direct chemical reaction of the surface of these alternative semiconductor materials has not yielded an insulator/semiconductor materials technology approaching the performance of silicon-silicon dioxide technology for electronic device applications. Part of this problem lies in the extreme reactivity of these semiconductor surfaces. Thus, even insulator deposition technologies, that by design seek to avoid interfacial reactions, result in some unintentional reactions which dominate the electronic properties of the deposited structure.